The present invention relates generally to the field of exploration and production of oil and other fossil fuels from a well, and more particularly, to a strong, lightweight aluminum riser apparatus, system and method of manufacturing same for use in offshore drilling and production.
Offshore drilling rigs, such as fixed platforms, jack-up platforms, floating and/or semi-submersible platforms, and dynamically positioned drill ships, are used in the production of hydrocarbons from under the floor of large bodies of water. A riser string is typically provided between the floating rig and the wellhead at the ocean floor. A conventional marine riser comprises a cylindrical pipe or column made of ferrous metal, e.g., steel, which is positioned vertically between the seabed and a drilling platform at the surface. The riser typically comprises a plurality of sections or joints connected end to end in a string between the surface and the wellbore.
A significant drawback to using riser constructed of steel is its high density and significant weight. A steel riser with adequate wall thickness to meet pressure requirements adds significant weight to the rig. The weight of the riser can substantially limit the payload capacity available for other necessary equipment and staff on the rig. Not only must each section be strong enough to carry the load of other sections, but also existing platforms can only carry a limited number of sections without exceeding their maximum load limit. A riser of inadequate strength can lead to failure of the equipment and can present a danger to the personnel on the platform.
Buoyancy modules are typically fitted to reduce the submerged weight. Top-tension is then applied to the riser string to prevent buckling of the string due to the weight of fluid in the bore of the riser and sea currents.
An increasing demand for drilling in greater depths of water has required additional riser pipe to be used in order to span the distance from the ocean floor to the floating platform. The added weight of the riser becomes a significant problem and a limiting factor at greater depths of water. Consequently, using a conventional steel riser at greater depths of water requires sacrificing even more valuable payload capacity to carry the necessary riser pipe. In addition, the added weight of a steel riser can increase the amount of fuel consumption and therefore increase costs of operations.
The use of a lighter weight material such as titanium has been described in the prior art. The high cost of titanium, however, is a significant disadvantage that renders its use impractical. Furthermore, the use of aluminum risers has not been previously executed with success, since common aluminum alloys lack the requisite strength properties.
A need has therefore arisen for a system, apparatus and method for drilling offshore that overcomes the limitations of the prior art. A riser composed of a material having a high strength-to-weight ratio and resistance to corrosion while reducing the overall weight of the drilling equipment would be a sorely needed improvement upon the prior art. Such an improved riser would allow offshore oil production at greater depths of water without increasing equipment costs, or jeopardizing the safety and security of the drilling operations.
Accordingly, the present invention provides an improved riser for use in offshore drilling operations. In accordance with a preferred embodiment of the present invention, a riser apparatus for use in offshore drilling comprises a plurality of riser sections coupled serially end-to-end, wherein each of the riser sections comprises a pipe having a first end and a second end, a first flanged coupling welded to the first end of the pipe, and a second flanged coupling welded to the second end of the pipe, wherein the pipe is constructed of an aluminum alloy having a strength-to-weight ratio greater than that of steel. The riser apparatus may optionally include one or more auxiliary lines providing hydraulic communication with a blowout preventer. The auxiliary lines may include without limitation choke and kill lines, hydraulic lines, and booster lines. In connection with the provision of auxiliary lines, telescoping joints may also be provided to allow for stretching of the riser with the movement of the floating rig due to factors such as ocean currents, waves, and the wind.
A preferred method of manufacturing the inventive riser is also disclosed, comprising the steps of welding a first flanged coupling to a first end of a pipe, welding a second flanged coupling to a second end of the pipe, and heating the welds at a temperature below the melting point of the welds sufficiently high to anneal the welds, wherein the material used for the welds is composed of an aluminum alloy having a strength-to-weight ratio greater than that of steel.
An object of the present invention is to provide a riser that is lighter than conventional steel riser, while still meeting pressure and strength requirements. By using a riser of a material having a high strength-to-weight ratio, excellent weldability characteristics, and resistance to corrosion, the present invention allows for a longer riser string as needed in offshore drilling operations in deeper waters.
Another advantage of the riser according to the present invention is that the lighter weight of the inventive riser allows for increased deck load capacity for equipment and operating supplies. The decreased weight of the inventive riser reduces the amount of top tension required and use of buoyancy modules. By reducing the amount of top tension, smaller tensioner units can be employed, thereby freeing even more deck space. The decreased weight of the inventive riser also reduces overall costs of the offshore drilling operations.
For a more complete understanding of the present invention, including its features and advantages, reference is now made to the following detailed description, taken in conjunction with the accompanying drawings.